Effect of Separation on the Fracture Surface of Pipeline Steels with Ferrite?Bainite Dual Phases During Drop Weight Tear Test

Yong Hwan Cho; Jaeeun Lee; Wung Yong Choo; Juseok Kang; Heung Nam Han

doi:10.1007/s12540-021-00999-4

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자료유형: 학술저널

저자정보: Yong Hwan Cho (Seoul National University) Jaeeun Lee (Seoul National University) Wung Yong Choo (Seoul National University) Juseok Kang (POSCO) Heung Nam Han (Seoul National University)

저널정보: 대한금속·재료학회 Metals and Materials International Metals and Materials International Vol.28 No.6

발행연도: 2022.6

수록면: 1,340 - 1,348 (9page)

DOI: 10.1007/s12540-021-00999-4

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The effect of separation on the fracture surface of ferrite?bainite dual-phase pipeline steels during a drop weight tear test(DWTT) was investigated via microstructural analysis and fracture surface examination. Two specimens (ST1 and ST2) weredesigned to have a dual-phase microstructure, with different ferrite and bainite fractions, by controlling the reduction ratioand holding time in the ferrite?austenite two-phase region during the thermomechanical process. Notably, ST2, which had alonger holding time for the retained austenite to absorb carbon, exhibited a harder bainite but softer ferrite phase comparedto ST1. The greater hardness difference between those phases in ST2 induced the strain incompatibility frequently evidentat the phase boundaries, resulting in lower ductility during the tensile test. Owing to the strain incompatibility, ST2 alsogenerated more separation over a wider temperature range during DWTT. At low temperatures, where the brittle fracture wasprominent, it was observed that ST2 generated separations on its fracture surface, whereas ST1 did not. These separationsformed a local shear lip around themselves, thereby obstructing cleavage fracture propagation from the notch. Hence, it wasconfirmed that ST2 had a higher DWTT shear area than ST1 over a temperature range near the ductile?brittle transitiontemperature (DBTT). As a result, the DBTT of ferrite?bainite dual phase steel could be improved by increasing the amountof separation during DWTT.

#Pipeline · Ferrite?bainite steel · Drop weight tear test · Separation · Fracture surface

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